Very interesting question. I might even give this as an exercise to my students! To my mind you need to start off by looking at the story again to see first off why the author ended it so suddenly the way he did and also how he foreshadowed the shocking ending. There is one key part that I think achieves this function of indicating the truth of what he has already done. This section comes after the sniper has won his duel to the death and the body falls from the roof:
The sniper looked at his enemy falling and he shuddered. The lust of battle died in him. He became bitten by remorse. The sweat stood out in beads on his forehead. Weakened by his wound and the long summer day of fasting and watching on the roof, he revolted from the sight of the shattered mass of his dead enemy. His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.
This is a highly fascinating passage for a number of reasons. It shows the sniper after the rush of adrenaline that has carried him through the last few hours now that it is over and it also shows his humanity. He realises the horror of war - that it splits nations apart and even families - as he is soon to discover.
Thus, to my mind, any dramatic monologue would try to capture some of these feelings of regret, severe remorse, and the lesson that the sniper has already learnt - that war creates schisms between countries, cities, families, and even brothers, with tragic and unforgettable consequences.